The present invention relates to lasers and especially to an unstable laser resonator using crossed Porro prism end reflectors and radially graded output reflectivity.
The fundamental components of a laser are a resonator, a laser medium, and an output coupling mechanism. A laser resonator basically consists of two resonator reflectors between which light travels back and forth. The region between the two reflectors is termed the resonator cavity. A laser gain medium within the cavity amplifies the light as it repeatedly traverses the cavity.
A stable resonator maintains the light beam to a limited diameter as it bounces back and forth, represented by closed ray paths between the resonator reflectors. Most commonly, the output beam is extracted from the resonator cavity by making one of the resonator reflectives partially transmissive. Stable resonator lasers are known for their ability to produce a light beam having good collimation and spatial coherence and having good "beam quality". However, because of the limited diameter of the beam, modest resonator lengths available in practical lasers, diffraction effects, and requirement for ray paths to be closed within a stable resonator, it is difficult to achieve simultaneously high-efficiency and diffraction-limited performance from a stable resonator laser.
An unstable resonator progressively increases the diameter of the light beam as it bounces back and forth between the two resonator reflectors. The portion of the beam whose diameter exceeds a certain value is generally extracted to form the output beam. Typical extraction techniques include deflecting the output beam away from the axis of the resonator cavity by an annular "scraper mirror" oriented at an angle to the axis, or else allowing the output beam to escape the cavity when its diameter exceeds that of one of the resonator reflectors. Unstable resonators do not impose requirements for closed ray paths, and are able to achieve excellent beam divergence while extracting energy efficiently from large gain volumes.
Lasers with low beam divergence and good beam quality, high efficiency, uniform beam profiles, and high reliability are required in industrial and military applications. Intrinsically low beam divergence or weak spreading of the laser beam is needed in order to strongly illuminate objects at large distances. Earlier stable resonator designs incorporated bulky, massive expansion telescopes at the output of the oscillator in order to reduce the beam divergence to an acceptable level. Unstable resonators have the desirable attributes of low divergence and high energy extraction efficiency. However, prior unstable resonator designs have not used alignment insensitive Porro prism end reflectors which greatly increase reliability of the system. In addition, they have not compensated for induced radial birefringence in the laser medium. Uncompensated birefrigence can result in low output energy, highly distorted spatial profiles, and damage in polarization coupled lasers.
Improved output characteristics using birefringence compensation and improved alignment stability with Porro prism end mirrors in stable polarization coupled resonators has been demonstrated in the past. However, stable resonators must trade off between efficiency, low beam divergence and good beam quality. A stable oscillator can operate with good energy extraction efficiency by allowing higher modes to exist. These same higher order modes however propagate at a much greater divergence. If the higher order modes are blocked to achieve low beam divergence, energy efficiency suffers. Unstable resonators inherently select the lowest divergence mode, even though the beam diameter in the laser medium is large. Thus, unstable resonators have an advantage over stable resonators in that they can be efficient and have low divergence.
Compared to "flat" mirrors, cross Porro prism end reflectors are made more insensitive to misalignment improving the reliability of the laser system. A crossed Porro prism resonator in a stable resonator is shown in U.S. Pat. No. 3,924,201 assigned to Applicant. In addition, techniques have been reported to compensate for thermally induced birefrigence in the gain media which require at least one Porro prism end reflector in a stable resonator. The present invention combines the best attributes of crossed Porro resonators employing birefrigence compensation with that of low magnification unstable resonators to achieve low beam divergence and high efficiency in an environmentally insensitive package.
Prior art unstable resonators or lasers can be seen in the Hoffmann, U.S. Pat. No. 4,491,950, for an unstable laser resonator having two spherical members and in the Pepper et al. patent, U.S. Pat. 4,803,696 for a laser with a grating feedback unstable resonator and in U.S. Pat. No. 4,787,092 to Gobbi et al. for a laser utilizing a negative branch unstable resonator. In the Trageser patent, U.S. Pat. No. 4,633,479, an alignment system for a confocal unstable laser resonator is shown while in the Morton, U.S. Pat. No. 4,423,511, an unstable waveguide laser resonator is shown. Other unstable laser resonators can be seen in the Komine et al. patent, U.S. Pat. No. 4,490,823, for injection of an unstable laser and in the Smith patent, U.S. Pat. No. 4,433,418, for an off-axis astigmatic unstable laser resonator. Birefringent plates for stabilization can be seen in the U.S. Pat. to Goodwin, 3,588,738, for a frequency stabilized laser and in the Lundstrom patent, U.S. Pat. No. 4,408,334, for a waveplate for correcting thermally induced stressed birefrigence in solid state lasers and in the Johnson, et al. patent, U.S. Pat. No. 4,935,932, for an apparatus using induced birefrigence to improve laser beam quality. These patents dealing with birefrigence as well as Applicant's prior patent using two Porro prism end reflectors are for stable resonators.
In contrast to these prior patents, the present invention deals with an unstable laser resonator which incorporates crossed Porro end reflectors for angular stability, birefringent compensation for improved beam uniformity, and a radially graded output coupling reflectivity used in conjunction with a polarizer to produce improved beam quality. The present method for producing the graded output coupling reflectivity is by incorporating a lens shaped waveplate. This combination results in a near diffraction limited output beam in which the efficiency is comparable to that of a conventional multimode laser having a many times diffraction limited output.